Process of producing oxygen



May 15, 1951 F. J. JENNY ET Al.

PRocEss oF PRonucING OXYGEN Filed Dec. 5, 1945 INVENTORS Patented Mayl5, `1951 PROCESS OF PRODUCING OXYGEN Frank J. Jenny, New York, N. Y.,andEdward G.

Scheibel, Nutley, N. J., assignors to Hydrocarbon Research, Inc., NewYork, N. Y.

Application December 5, 1945, Serial No. 632,859

This invention relates to the production of oxygen by the liquefactionand rectification of air, and more particularly to the operation of theusual two-stage rectification column and associated heat exchangers.

All temperatures herein given are in degrees F. and pressures are in.pounds per square inch gauge.

Oxygen is commonly` produced by partial liquefaction of air andrectification at low temperatures; preferably rectification is conductedin two stages at diiferentpressures. The refrigeration necessary forliquefaction is supplied to the air after it has been compressed andwatercooled to approximately room temperature, by indirect heat exchangewith the effluent products of rectification. Howevenan additional amountof refrigeration must be, supplied to compensate for cold lossesresulting from the difference in enthalpy between the incoming air and.the outgoing products of rectification and for heat leaks into thesystem. Methods of supplying this refrigeration heretofore used, involvecompressing at least a portion of the incoming air to pressures as highas 300D pounds and expanding with or without the performance of work toproduce a temperature drop; or compressing all the inf coming air toabout 600 pounds and after the air has been .partially cooled by theproducts of rectiiication expanding a portion of the air. These methodsare wasteful from the standpoint of compressor energy and require agreat deal of equipment in the form of extra compressors, intercoolersand expanders.

For economical operation it is essential to recover the cold content ofthe outgoing products of rectification. This is usually accomplished bypassing these products in heat transfer relationship with the incomingair. In older systems in order to avoid deposition of frost and solidcarbon dioxide in the tubular countercurrent heat exchangers throughwhich the air is passed in indirect heat exchange relation with theoutgoing products of rectification, the air is treated in driers andcaustic scrubbers to remove water and carbon dioxide prior to admittanceof the air into the heat exchangers. Even with this treatment theexchangers had to be thawed out regularly to remove the frost (whichterm is used in a generic sense to` include both snow and ice) whichcaused stopping up of th@ apparatus.

More recently it has been suggested to use cold accumulators orregenerators (hereinafter referred to as heat exchangers) of large 4cold6 Claims. (Cl. 62-175 .5)

absorbingv capacity through which the. warm incoming air and the coldproducts` of rectification are alternately passed with periodicallyreversed operation so that streams of warm air are flowed through thesame packing filled spaces that the cold separated oxygen and nitrogentraversed during the previous step in the process, the high boilingimpurities deposited. in these spaces during the passage of airtherethrough being. removed by sublimation during the subsequent flow ina reverse direction of the products` of rectification. The use ofthesereversing heat exchangers in a process in which the air is.compressed. to relatively high pressure results in more costlyoperationV from the standpoint of horsepower requirements because uponevery reversal, which may take place every three minutes, the volume ofcompressed air in the heatexchangers is lost and must be again replaced.

In copending application Serial No. 632,858 filed December 5, 1945,there is disclosed and claimed a. process for producing oxygenbyliquefaction and rectification of air involving the flow of air at about'l0` to about 85 pounds at a temperature of about to about 110 li'.through the heat` exchange paths Yof two` orY more reversing heatexchangers in series, `each exchanger containing two other paths throughwhich` are passed respectively streams of oxygen and nitrogen productsof rectification in heat exchange relation with the air passingtherethrough. The airstream flowing from the rst exchanger to the secondis refrigerated, the amount of cold thus introduced into the processbeing adequate` to compensate for cold. losses resulting from thedifference in enthalpy between the air introduced into and the productsof rectication withdrawn from the process and for heat leaks into thesystem. The temperature conditions in the first exchanger are such thatsubstantially all: moisture present in the air is removed therefrom inthe` form of frost. The temperature conditions in the second exchangerare such as to effect substantially complete removal of carbon dioxidefrom theiair in its passagetherethrough.

At the colder end of the second exchanger where the oxygen and nitrogenproducts of rectication enter and air leaves the exchanger, there ismaintainedbetween these products of rectification and the countercurrentstream of air a temperature difference in the range of about 5 to about10 F., preferably about 6 to about 8 F. This temperature difference isthe difference between the temperature of the air and the weightedaveragetemperature of the products of 3 rectification, all temperaturesbeing taken at the colder end of the second exchanger. For the Ypurposes of this invention, the weighted average temperature of theproducts of rectication is calculated by multiplying the temperature ofthe oxygen product stream by the volume percentage of the stream basedon the combined volume of the products Vof rectification and addingthereto the corresponding ligure obtained by multiplying the temperatureof the nitrogen product stream by its volume percentage. Thus, forexample, if the'rectication system is operated to produce two streams ofsubstantially pure oxygen and pure nitrogen, the weighted averagetemperature of the two streams would be approximately the sum of of theoxygen stream temperature and 80% of the nitrogen stream temperature.Periodically the flow of air and nitrogen Ythrough their respectivepaths in the two exchangers is reversed so that upon reversal the airflows through the paths in the two exchangers through which during thepreceding step the nitrogen had passed and the nitrogen ilows throughthe paths in the two exchangers through which had previously passed theair. The nitrogen removes, by sublimation, the carbon dioxide depositedduring the preceding step in the second exchanger andthe frost depositedduring the preceding step in the rst exchanger.

Operating in this manner complete purging of carbon dioxide is attainedupon each reversal of ilow; Likewise complete purging oi frost isobtained so that the equipment may be operated continuously.

This invention is in the nature of an improvement on the inventiondisclosed and claimed in the aforesaid copending application.. It is anobject of this invention to effect removal of incondensibles, such ashydrogen, helium and neon, from the rectification system withoutreduction in the yield of oxygen recovered in the process.

panded to cool the same and the cold thus produced imparted to therectification products entering the low-pressure stage and preferablyalso to the air entering the high-pressure stage. Periodically the ilowof air and nitrogen is reversed through their respective paths in thetwo zones, the air upon reversal flowing through the paths through whichhad previously owed the nitrogen and the nitrogen ilowing through the`paths through which had previously flowed the air, whereby upon eachreversal the nitrogen substantially completelyV removes the carbondioxide A further object is to increase the efficiency of Y theoperation of the rectication system. Other objects and advantages ofthis invention will be apparent from the following detailed description.

In accordance with this invention a stream of air is passed through apath in two zones in series, each of the zones containing at least threepaths in heat exchange relation with each other and streams of oxygenand nitrogen rectification products are passed through the other twopaths inthese zones in heat exchange relation With/the air passingtherethrough. One of the streams is cooled in its now between the firstand second zones to a temperature suilicient to supply to the system thenecessary cold to compensate for cold Vlosses resulting from thediierence in enthalpy Abetween the incoming air and the deposited in thesecond Zone during the preceding step of the process.

In the preferred embodiment of the invention, a minor portion of thenitrogen withdrawn from the high-pressure Ystage of the rectificationsystem is passed through the second zone where the nitrogen is heated bythe air stream flowing to the high-pressure stage of the rectificationsystem. The heated nitrogen is then mixed with the remainder of thenitrogen withdrawn from the high-pressure stage of the recticationsystem thereby increasing the temperature of the nitrogen, preferablynot more than 20 F., and the mixed nitrogen stream thus introduced intothe expander at a temperatureV such that no liquid nitrogen is formedwithin the expander with consequent improvement in the Vefdciency oi theoperation of the expander. i f

In the preferred embodiment illustrated inthe drawing, the single iigureof which illustrates diagrammatically a preferred layout of apparatusfor practicing the process of this invention, the yequipment shownforthe practiceY of the process involves a pair of heat exchangers havingan ethylene refrigeration system for refrigerating the air and thepresent description will be confined to the present illustratedembodiment of the invention. It will be understood, however, that theprocess may be carried out in other equipment, for example, each of thetwo exchangers in series may be replaced by two or more smallerexchangers placed in series and/or parallel, if desired, although thisis objectionable from the standpoint ofincreasing construction costs, orthe number of heat exchange paths in each exchanger may be increasedover the 3- outgoing products Vof rectication and for heat leaks'intothe system. The temperature difference between the temperature of theair leaving and the temperature of the rectification products enteringthe second Zone is maintained withinV j the range of from about 5 toabout 10 F. and the exit temperature of the air leaving this zone issuch as to eiect substantially complete removal of carbon dioxide fromthe air in its passage through its path in this zone. From this second'zone the air is'passed tothe high-pressure stage of a two-stagerectification system in indirect heat exchange relation withrectification prod- Y ucts from the low-pressure stage of the system.

' part of this invention and as it may beV ofY anyV well-known type, itisbelieved further descrip-- path construction shown in the drawing, orother refrigeration systems may be employed in lieu of the ethylenesystem. Hence, the scope of the invention is not confined to theembodiment herein described. Y

In the drawing reference character .lil ndicates a heat exchangerwhiclimay be Yof any well-known type. In the embodiment shown in the drawingsit consists of a single shell in which are provided three paths, namely,interior path li through which flows in one and the same directionthroughout the operation of the exchanger the oxygen `product ofrectification.

YPaths I2 and I3 are provided within the shell Y oi the exchangerYthrough which periodically flow air and the nitrogen product ofrectication in heat exchange relation with each other and with theoxygen. The heatexchanger has in each of the paths suitable finsofheat-conducting material, e. g., copper, promoting rapid and efficientheat exchange between the gaseous media ilowing therethrough. As theconstruction Vof the heat exchanger per se does not Vform tion thereofis unnecessary.

The flow of the air'and nitrogen through their respective. paths isperiodicallyf reversede so, that during, one step of` the; processairflows through path I2 and nitrogenthroughpath. I3, and upon reversal,during the succeeding step air flows through path I3` and4 nitrogenthrough path I2.

Reversal of now is` accomplished by suitably positioning the compoundreversing valves I4 and I5 which may be of any well-known type. Valve I4is disposed in the pipe line system consisting of air inlet pipe I5leading into valve I4, and pipe lines I'I and I8 leading from the valveto cooling paths I2 and I3, respectively. At the base of the heatexchanger If lines I!)` and are positioned leading from paths I2` andI3, respectively, to the Valve I5.

A second heatexchanger 2I is provided in the form cf a shell havingtherein paths 22, 23 and 24 provided with iins to promote heat exchangeas in the case; of the exchanger Ill. Path 24, is.` the path throughwhich the oxygen product I of rectification flows :from` therectification system hereinafter described to a pipe line 25. whichcommunicates with path I I of heat exchanger I3'. The base portions ofpaths 22 and 23 of heat exchanger 2| communicate with pipe lines 26 and2, respectively, which are communicably co-nnected with a compound valve28 which may be of the same type as4 valves I4 and I5. At the upperportions paths 22 and 23 communicate respectively with lines 29 and 33which in turn communicate with a compound reversing valve 3l which maybe oi the same type as the other reversing valves.

Reversing exchangers II! and2| may be placed in vertical, horizontal orany other desired position. Likewise, when these exchangers are arrangedvertically, the colder end may be above or below the warmer end.

A refrigeration system 32 of any well-known construction. for supplyinga refrigerating medium, such as ethylene, or carbon tetraiiuoride isprovided for cooling either the nitrogen flowing from heat exchanger 2Ito heat exchanger IB, or the air flowing from heat exchanger IE! to heatexchanger 2 I. This refrigerating system operates t;

to cause the now of the refrigerating medium in indirect heat exchangerelation with the ntirogen or air to be cooled, the rate of flow andtemperature of the various media being so controlled that enough cold isintroduced by refrigeration, I

at this point in the process, to compensate; for cold losses resultingfrom the difference in enthalpy between the incoming air and. theoutgoing products oi rectification and for heat leaks` into the system.In the preferred embodiment of the inventionV the air leaving the heatexchanger II! is refrigerated to cause a drop oi about 5 to about l0" F.in its temperature; this has been found adequate for the purposes abovestated. Refrigeration of the air is accomplished by causing it to flowthrough pipeline, 33 which passes through the refrigerator 32 inindirect heat exchange with the` refrigerant and comcunicably connectsvalves I5 and 28. Line 9 is the nitrogen line between the two heatexchangtem. The. coldexpanded: air thus produced` may be introduced.into the nitrogen stream entering theheat exchanger Iii therebysupplyingthe necessary cold to compensate. for cold losses resultingfrom the difference in enthalpy between. the incoming air and theoutgoing products of rectification and for heat leaks into the system.This latter method has the disadvantage that. it involves a loss ofapproximately 7% of the oxygen content of the air introduced. into thesystem. On. the other hand it has the advantage that it eliminates thenecessity for using a refrigeration system for cooling either thenitrogen` or air, which system is more cumberf some and expensive` inconstruction and, operation than an expander of the type suitable forexpending. a relatively small amount of air ata relatively low pressure,e. g'., 70-85 pounds gauge.

With the arrangement of values and piping shown flow of nitrogen and.air through heat exchangers III and 2| may be periodically reversed, sayevery three minutes, so that during an initial period of operation air'lows through heat exchange path I2, through line I3, valve I5,refrigeration system 32A by way of line 33, valve 2.3, line 25, coolingpath 22 in heat exchanger 2i, ypipe line 29, valve 3l and thence to line34 leading through the non-reversing heat exchanger 35 tov therectication system hereinafter described. At the same time, nitrogenflows through pipe line 35 leading from the non-re versing heatexchanger 35 into valve 3l, line 33, through path 23 in heat exchanger2l, through line 21, valve 28, line 9, valve I5, pipe line 23; path I3;in heat exchanger I0, leaving` this. path through pipe line- I8 andpassing through valve I4 to the atmosphere or other suitable disposalpoint. Upon reversal (as shown by dotted arrows and valve settings), theair flows through valve I4, line I8, path I3, pipeline 23, valve I5,refrigeration system 32 by way of line 33, valve 28, pipe line 2, andthence through the cooling path 23, leaving` this cooling path throughpipe line 3l] and passing through valve 3| and pipe line 34A into thenon-reversing exchanger 33. At the same time, the nitrogen Hows fromheat exchanger 35. through pipe line 36. into valve 3l, thence throughpipe line 29, path 22 in heat exchanger 2|, pipe line 26, valve 28, line3, valve I5, line I9 into path I2, thence through line I? into valve I4and thence to the atmosphere or other suitable disposal point.

The rectiiication system comprises a two-stage rectication column 37,the lower section 38 of which is operated at a pressure of about l2pounds gauge and the upper section 39 of which is operated ata pressureoi from about 4 pounds to about l0 pounds gauge, preferably at about 5pounds gauge. This column as is customary is provided with rectificationplates of the bubble-cap or other desired type. The lower section 38 ofthe column 31 communicates with a condenser 4U and has a liquidcollecting shelf 4I disposed immediately below thecondenser i3 forcollecting liquid nitrogen. Pipe line 42 leads from this shelf 4I to anon-reversing heat exchanger 43 which in turn communicates through apressure reducing valve 44 with the tcp por tion of the upper section 33as indicated by the reference character 45. Condenser 43 acts as areboiler for the upper section 39 of the column 31. Y

From the .base portion of the lower section-i 38 a pipe line 4B Vfor theflow of crude oxygen (con .f taining approximately 49%- oxygen) passesto a non-reversing heat exchanger ,41 which communicates with pipe line48 havingV a pressure reducing valve 49 therein with the 10W pressuresection 39 at an intermediate point indicated by the reference character50. Line I leads from the top of the condenser 40 and has a regulatingvalve 52 therein. This line communicates with an expander 53 whichdischarges by way'of. line 53a into line 54 hereinafter described.Preferably, there is also provided a branch line 53 having a regulatingvalve 59 and leading to a path 6U disposed in heat exchanger 2| inindirect heat exchange relation with the oxygen, nitrogen and airpassing through the other three paths in this exchanger 2 I'.V A line 6|leads from path 60 back to line 5|. Regulating valves 52 and 59 disposedin lines 5I and 58, respectively, regulate the portions of the nitrogenstreamllowing from the condenser 40 which are passed directly toexpander 53 and indirectly through path Vlill of exchanger 2|. Y

By the arrangement of lines hereinabove described Va minor portion `ofthe total nitrogen introduced into the process passes through line 5|and, preferably, of the portion thus Withdrawn a minor portion, sayabout 10%, passes through linev 58, path 69 and line 6| entering line 5|where it mixes with the remainder of the nitrogen withdrawn from thecondenser 40. IThe portion of nitrogen p-assing through path 60 iswarmed up by indirect heat exchange, and by mixing with the remainder ofthe nitrogen, the stream entering expander V53 is at a temperaturesuilcient to avoid condensation or formation of liquid nitrogen in theexpander. In a preferred embodiment of the invention from about 1% toabout by volume of the total nitrogen introduced into the process andcontaining incondensibles, such as hydrogen, helium and neon, is passedthrough line 5| and of this quantity about 10% by volume passes throughheating path 60 and 90% by volume continues through line 5|.

The nitrogen stream refrigerated as a result of the expansion flows fromthe expander 53 to a line 53a which meets line 54 conveying the nitrogenstream leaving the top of low-pressure section 39. The mixture thenflows through heat exchanger 43 in indirect heat exchange relation withthe nitrogen passing through this exchanger and thereafter flowingthrough reducing vvalve 444 into the top of low-pressure section 39.From Vheat exchanger 43 the mixed nitrogen stream flows through line55'into.and through heat exchanger 41 where it flows in indirect heatexchange relation with the crude oxygen flowing therethrough tolow-pressure section 39. From the heat exchanger 41 the mixed nitrogenstream passes through line 59 into and through heat exchanger 35 whereit passes in indirect heat exchange relation with air flowing into andfrom this exchanger, by way of line 34. From the heat exchanger 35 thenitrogen stream flows through line 35 into a compound valve 3|, thencethrough path 22 or 23, as the case may be, of heat exchanger 2|, throughvalves 28 and I5 connected by line 9, then through path I2 or I3 of heatexchanger I0V and finally through compound valve I4 tothe atmosphere;the flow throughV path I2 0r VI3 of heat exchanger II) depending uponthe setting of valves |4'and `I5 and the lowthrough path 22 or 23 ofheat exchanger 2| depending upon the setting of Yvalves 28 and 3| ashereinabove described in connection with the operatio of these reversingheat exchangers.

, The heat exchangers 35, 43 and 41 and the twostage fractionatingcolumn 31 may be of any conventional type. Two separate fractionatingco1- umns, suitably interconnected may be used inY place of thetwo-stage column 31 shown. It will be understood that the equipmentthroughout is heat insulated to minimize loss of cold.

One example of the'operation of the process of this invention isdescribed below. It will be understood this examples given for purposesof exemplication only and the invention is not limited thereto.

Air under pressure of about 75 pounds gauge and temperature of about 100F. is supplied through lineIS, valve I4 and line I1 to heatV exchanger|0, flowing through path I2 in which it is cooled to a temperature of134.5 F. The air then flows in indirect heat exchange relation withethylene in the refrigeration system 32 and is cooled thereby to atemperature of 142 F., then passes through path 22 of the heat exchangerY 2| leaving this path at a temperature of 275 F.

Substantially all moisture is removed in the form of frost in path I2 ofheat exchanger Id and all carbon dioxide is removed in solidied form inpath 22 of heat exchanger 2 I. The air then flows through the heatexchanger 35 in heat exchange relation with nitrogen and entershigh-pressure column 36 at a temperature of 278 lFrand aV pressure of 72pounds. f;

Crude oxygen at a temperature of 280 F. and a pressure of 72 poundsleaves the base of column 39, flows throughV heat exchanger. 41 wher itstemperature is reduced to 289 F. and upon flow through the pressurereducing Valve '59 is ashed, entering low-pressure column 39 at atemperature of 310 to 315 F. and a |pressure of 5 pounds. Pure oxygen iswithdrawn through line 51 at a temperature of 292.5 F.

and a pressure of 5 pounds and'rfiows through a path 24, its temperaturebeing increased to 146 F., the oxygen at this temperature entering pathII of heat exchanger I0 and being withdrawn from this path at atemperature of 90 F. and at a pressure of 1 pound.

Nitrogen at a temperature of Vabout 286.5 F. and a pressureroif 72pounds-in amount equal to 12.5% by volume of the totalrnitrogenintroduced into the process is withdrawn through line 5|. Of thenitrogen owing through line 5I, 10%

' passes through line 58 and path 69, its tem-pera- Y through exchanger43 in indirect heat exchange relation with nitrogen employed as VreiiuxV.in column 39, its temperature being thereby increased 'to 306 F. whilethe Vtemperature of nitrogen flowing through line 42 (pressure of Y '12pounds) Vand exchanger 43 is reduced to 300 Ff This nitrogen byexpansion through valve 44 has its pressure reduced to 5 pounds and itstemperature to 315.5 F. The nitrogen product of rectification then owsthrough heat exchanger 41 Where its temperature is increased to 293 F.The crude oxygen stream owing through exchanger 41 is thereby cooledfrom a temperature of 280 F. to a temperature of -:289 F. The Anitrogenthen flows through exchanger 35 in heat exchange relation with the fair,the nitrogen stream temperature Abeing thereby .increased to 279 atwhich temperature it Venters the heat exchanger 2l, `:flows therethroughand is thereby heated `to 146 F. At the colder end of the heat exchanger2l., the nitrogen and the oxygen streams have a `weighted averagetemperature of nearly 282 F. while at this point the air is at atemperature Yof 275 F. A temperature differenceoi about '7 F. istherefore maintained. It will be noted that at the point where the airenters fand the `oxygen and nitrogen leave the heat exchanger 2l thediierencein temperature is approximately 4 F., the air being at atemperature of 142 F. and the oxygen and nitrogen at 146 F.

From heat exchanger -2I the nitrogen flows through heat exchanger Swhere it is `heated to 90 F. The nitrogen at `this temperature and a`pressure slightly above atmospheric, say 1 pound gauge, may `he ventedto the atmosphere thereby venting the incondensibles, such as hydrogen,helium and neon, removed .from the .high-pressure stage of therectification system.

'Ihe desired -oxygen product is withdrawn through line 51 `at atemperature of -292.5 F.

and `pressure of pounds, flows `through heat exchanger 2l where itstemperature is increased to -146 F. and then through heat `exchanger l0where its temperature is `increased to 90 F. The oxygen leaves exchangerI0 at a pressure of 1 pound.

Upon reversal (as shown by dotted arrows and valve'setting-s) which maytake place every three minutes, the air flows through paths IS and 23,respectively, of heat exchangers l0 and 2l and nitrogen hows throughpaths VI2 land V22respectively,.of heat exchangers i0 and 1l. The now isotherwise substantially the same and the temperature and pressureconditions remain the same. The nitrogen in its now through path 2-2 ofheat exchanger 2| removes by sublimation the carbon dioxide deposited inthis `path by the air during the preceding step. Likewise, the nitrogenin its ow through path l2 of Lheat exchanger I removes from thispath thefr-ost deposited 'therein from the air during the preceding step. Thusin the continued operation upon each reversal the nitrogen effectsremoval of the carbon dioxide and frost deposited in the paths throughwhich the air had passed during lthe preceding step of the process.

Operating in accordance with this invention it is found possible torecover substantially 100% of the oxygen content `introduced into therectification system in the form of substantially pure oxygen and at thesame 'time effect continuous Ip1,1i'ging from the high-pressure stage ofthe rectification system of the incondensible constituents, such ashydrogen, helium and neon. .Further the purging is carried out so as toincrease the eiliciency of the rectication system in that the nitrogenstream containing the incondensibles is expanded and therefrigerationthus produced employed to cool thereflux oxygen and nitrogen introducedinto the low-pressure stage and the air introduced `into thehigh-pressure stage.

The expressions reversing the low of `air and nitrogen and "reversal areused herein in the sense commonly employed in this art, namely, to meanthe switching o1" the ow oi two streams, for example, the air and thenitrogen streams, so that upon each ,reversal the air owslthrough 10"the rpath through which had previously flowed the nitrogen, and thenitrogen flows through `the path through which had previously iiowed theair.

It will be noted this invention provides a process for producing oxygenof high ,purity without the use of chemical agents, which process may`loe operated continuously `over a long period of time withoutVsluit-downs for the `purpose of removing solid carbon dioxide or frost,which process is economical to operate, particularly in that Vtherefrigeration which must be supplied to compensate for cold lossesresulting from the difference in enthalpy 'between incoming air and. theoutgoing products of rectification and for heat leaksiinto the system issupplied at a point .in the process where the temperatures arerelatively high so that it can be supplied `eiiiciently andeconomically, and which process effects continuous purging from thehigh-pressure stage of the rectification system of the incondensibleconstituents, such as hydrogen, helium and neon, so as to increase theeiiiciency of the rectification system.

Since certain changes may be made in carrying out the above processwithout departing from the scope of the invention, it is intended thatall matter contained in the `above description shall be interpreted asillustrative and not in a. limiting sense.

What is claimed is:

l. A process for'producing `oxygen by the lique- `faction andrectification of air, which vcomprises passing a stream of air throughAa path in two zones in series, each of said zones containing at leastthree paths in heat exchange relation with each other, passingrespectively streams of oxygen and nitrogen. products of rectificationthrough two other paths in said zones in heat `exchange relation withthe'air passing therethrough, cool- `ing at least one of `said streamsduring its now between the first zone andthe second Zone, withdrawingthe air from the second zone at a temperature such that substantiallyall carbon dioxide in said air has been removed from the air in itspassage through its path in said second Zone, maintaining thetemperaturediference between the temperature of the air leaving and `theweighted average temperature of the nitrogen and oxygen entering saidsecond Zone so that it ifalls within the range or from about 5 to about10 F., passing the air from said second zone to the high-pressure stageof a two-stage rectication system in indirect heat exchange relationwith a rectication product from the low-pressure stage of said system,withdrawing a minor portion of the nitrogen from thehigh-pressure'stage, expanding the nitrogen thus withdrawn to cool thesame, imparting the cold thus produced to the rectification productsentering the low-pressure stage, periodically reversing the flow of airand nitrogen through their respective paths in said zones, the air uponreversal flowing through the paths through which had previously flowedthe nitrogen and the nitrogen owing through the paths through which hadpreviously flowed the air, whereby upon each reversal the nitrogensubstantially completely removes the carbon dioxide deposited in saidsecond zone during the preceding step of the process.

2. A process for producing oxygen by the liquefaction and rectiicationof air, which comprises passing a stream of air at about '70 pounds toabout pounds gauge and a temperature of about '70 to about 110 F.througha path in two least three paths in heat exchange relation witheach other, passing respectively streams of oxygen and nitrogen productsof Yrectification through two other paths in said zonesl in heatexchange relation with theair passing therethrough, cooling at least oneofY said streams during its ow between the rst zone and the second zone,the amount of cold thus introduced into the process being adequate tocompensate for cold losses resulting from the difference in enthalpyYbetween the air introduced into and the products of rectificationwithdrawn from the process and for heat leaks into the system,withdrawing the air vfrom the second' zone at a temperature such thatsubstantially all carbon dioxide in said air has been removed from theair in itsV passage through its path in said second zone, maintainingthe temperature difference between the temperature of the'air leavingand the Weighted average temperature of the nitrogen and oxygen enteringsaid second zone so that it falls within Vthe range of from about toabout 10 F., passing the air from said second zone to the high-pressurestage of a two-stage rectication system in indirect heat exchangerelation with a rectification product from the low-pressure stage ofsaid system, withdrawing a minor portion of the nitrogen containingincondensible gases from the high-pressure stage, expanding the nitrogenthus withdrawn to cool the same, passing the expanded nitrogen in heatexchange relation with nitrogen and oxygen supplied as reflux to thelow-pressure stage and with air supplied to the high-pressure stage, andperiodically reversing the iiow of air and nitrogen through Y theirrespective paths in said zones,.the air upon reversal flowing-throughthe paths in the two zones through which had previously flowed thenitrogen and the nitrogen flowing through the paths through which hadpreviously owed the air, whereby upon each reversal the nitrogensubstantially completely removes the carbon dioxide deposited in thesecond Zone during the preceding step of the process.

3; A process for producing oxygen by the liquefaction and rectificationof air, which comprises passing a stream of air at about '70 pounds toabout 85 pounds gauge and a temperature of about 70 to about 110o F.through a path in two zones in series, each of said Zones containingthree paths in heat exchange relation with each other, passingrespectively streams of oxygen and nitrogen products of recticationthrough the two other paths in said zones in heat exchange relar tionwith the air passing therethrough, cooling at least one of said streamsduring its flow from one zone to the other, the amount of cold thusintroduced into the process being adequate to compensate for cold lossesresulting from the difference in enthalpy between the air introducedinto and the products of rectification withdrawn from the process andfor heat leaks into the system, withdrawing the air from the second zoneat a temperature such that substantially all carbon dioxide in said airhas been removed from the air in its passage through its path in saidsecond zone, maintaining the temperature difference between thetemperature of the air leaving and the weighted average temperature ofthe nitrogen and oxygen entering said second Zone so that it fallswithin the range of from about 5 to about 10 F., passing the air fromsaid zones to the high-pressure stage of a two-stage rectificationsystem in indirect heat exchange relation with a rectification productfrom theVlow-pressure stage of said system, withdrawing from thehigh-pressure stage about 1% to 15% of the Y total nitrogen introducedinto the process, said nitrogen containing incondensible gases, heatingapproximately 10% of the nitrogen thus withdrawn, mixing the heatednitrogen with the re'- maining 90% of the nitrogen thus withdrawnthereby yielding a nitrogen stream having a temperature suflicientlyhighto avoid the formation of liquid nitrogen infthe expander,Vexpanding said nitrogen stream, passing the expanded nitrogen in heatexchange relation with oxygen and nitrogen supplied as reilux to thelow-pressure stage and with the Vlair supplied to the highpressurestage, and periodically Vreversing the flow of air and nitrogen throughtheir respective Y pathsV in said two zones, the air upon reversalflowing through the paths in the two YZones through which had previouslyflowed the nitrogen and the nitrogen iiowing through the paths in thesaid two zones through which had previously flowed the air, whereby uponeach reversal the nigtrogen substantially completely removes the carbondioxide deposited in the second Zone during the preceding step of theprocess.

4. A process for producing oxygenY by the liquefaction and rectificationof air, which comprises passing air at about 70 to about 85 pounds gaugeand a temperature of about 70 to about 110 F. through a path in a Zonecontaining three paths in heat exchange relation with each other,flowing oxygen and nitrogen products of rectification respectivelythrough the` other two paths in said Zone in heat exchange relationVwith the air, the air thus being cooled to a temperature sufficientlylow to deposit out as frost substantially all the moisture in the air,refriger-ating the air leaving the rst zone to lower its temperatureabout 5 to 10 F., then further cooling the air by flowing it through apath in a second zone containing three paths, flowing oxygen andnitrogen products of rectication respectively throughY the other twopaths in said second zone in heat exchange relation with the air therebycooling the air to a temperature suiiiciently low to deposit outsubstantially all the carbon dioxide in the air, the differentialbetween the temperature of the air and the weighted average temperatureof the oxygen and nitrogen at the colder end of said second zone beingwithin the range of about 5 to about 10 F., passing the air from saidzones to the high-pressure stage of a two-stage rectification system inindirect heat exchange relationwith a rectification product from thelowpressure stage of said system, withdrawing from 1% to 15% of thetotal nitrogen introduced into the process from the high-pressure stageof the system, said nitrogen containing incondensible gases, expandingthe stream of /nitrogen thus withdrawn, mixing the expanded nitrogenwith a stream of nitrogen withdrawn from the lowpressure stage andpassing the resultant nitrogen in heat exchange relation with nitrogenand oxygen fed to the low-pressure stage and air fed to thehigh-pressure stage, and periodically reversing theY flow of air andnitrogen through their respective paths in the said two zones, the

air upon reversal flowing through the paths inA upon each reversal thenitrogen substantially.v

assasse faction and rectification of air, which comprises passing astream of air through a path in two zones in series, each of said Zonescontaining three paths in heat exchange relation with each other,passing respectively streams of oxygen and nitrogen products ofrectification through the two other paths in said zones in heat exchangerelation with the air passing therethrough, thereby cooling the airleaving said second zone to a temperature suflicient to removesubstantially all carbon dioxide therefrom the carbon dioxide thusremoved being deposited in said second zone, cooling at least one ofsaid streams during its flow from one zone to the other, the amount ofcold thus introduced into the process being adequate to compensate forcold losses resulting from the difference in enthalpy between the airintroduced into `and the products of rectification withdrawn from theprocess and for heat leaks into the system, passing the airfrom saidsecond zone to the high-pressure stage of a two-stage rectificationsystem, lwithdrawing from said high-pressure stage nitrogen containingincondensible gases, heating by indirect heat exchange with the airpassing through said second zone the nitrogen thus withdrawn to atemperature suciently high to avoid the formation of liquid nitrogenupon expansion of said nitrogen, expanding said nitrogen, passing theexpanded nitrogen in heat exchange relation with oxygen and nitrogensupplied as reflux to the low-pressure stage of said rectificationsystem, `and periodically reversing the low of air and nitrogen throughtheir respective paths in said two zones, the air upon reversal flowingthrough the paths in the two `zones through which had previously flowedthe nitrogen and the nitrogen flowing through the paths in the said twozones through which had previously flowed the air, -whereby upon eachreversal the nitrogen substantially completely removes the carbondioxide deposited in the second zone during the preceding step of theprocess.

6. A process for producing oxygen by the liquefaction and rectificationof air, which comprises passing a stream of air at about 70 pounds toabout 85 pounds gauge and a temperature of about 70 to about 110 F.through a path in two zones in series, each of said zones containingthree paths in heat exchange relation with each other, passingrespectively streams of oxygen and nitrogen products of rectificationthrough the two other paths in said zones in heat exchange relation withthe air passing therethrough, thereby cooling the .air leaving saidsecond zone to a temperature sufficient to remove substantially all 14carbon dioxide therefrom the carbon dioxide thus removed being depositedin said second zone, cooling at least one of said streams during itsflow from one zone to the other, the amount of cold thus introduced intothe process being adequate to compensate for cold losses resulting fromthe difference in enthalpy between the air introduced into and theproducts of rectification withdrawn from the process and for heat leaksinto the system, passing the air from said second Zone to thehigh-pressure stage of a two-stage rectication system in indirect heatexchange relation with a rectification product from the 10W- pressurestage of said system, withdrawing from the high-pressure stage fromVabout 1% to about 15% of the total nitrogen introduced into the process,said nitrogen containing incondensible gases, heating a minor portion ofthe nitrogen thus withdrawn by indirect heat exchange with all of theair passing to the high-pressure stage of the rectification system,mixing the heated nitrogen with the remainder of the nitrogen thuswithdrawn, thereby producing a nitrogen stream having a temperaturesufficiently high to avoid the formation of liquid nitrogen uponexpansion, expanding said nitrogen stream, passing the expanded nitrogenin heat exchange relation with oxygen and nitrogen supplied as reflux tothe lowpressure stage and with the air supplied to the high-pressurestage, and periodically reversing the flow of air and nitrogen throughtheir respective paths in said two zones, the air upon reversal flowingthrough the paths in the two zones through which had previously flowedthe nitrogen and the nitrogen flowing through the paths in the said twozones through ywhich had previously flowed the air, whereby upon eachre- -versal the nitrogen substantially completely removes the carbondioxide deposited in the second zone during the preceding step of theprocess.

FRANK J. JENNY. EDWARD G. SCHEIBEL REFERENCES CITED The followingreferences are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,539,450 Wilkinson May 26, 19251,607,322 Van Nuys Nov. 16, 1926 2,057,804 Twomey Oct. 20, 19362,422,626 Koehler June 17, 1947 2,460,859 Trumpler Feb. 8, 1949 FOREIGNPATENTS Number Country Date 373,918 Great Britain June 2, 1932 469,943Great Britain Aug. 3, 1937

1. A PROCESS FOR PRODUCING OXYGEN BY THE LIQUEFACTION AND RECTIFICATIONOF AIR, WHICH COMPRISES PASSING A STREAM OF AIR THROUGH A PATH IN TWOZONES IN SERIES, EACH OF SAID ZONES CONTAINING AT LEAST THREE PATHS INHEAT EXCHANGE RELATION WITH EACH OTHER, PASSING RESPECTIVELY STREAMS OFOXYGEN AND NITROGEN PRODUCTS OF RECTIFICATION THROUGH TWO OTHER PATHS INSAID ZONES IN HEAT EXCHANGE RELATION WITH THE AIR PASSING THERETHROUGH,COOLING AT LEAST ONE OF SAID STREAMS DURING ITS FLOW BETWEEN THE FIRSTZONE AND THE SECOND ZONE, WITH DRAWING THE AIR FROM THE SECOND ZONE AT ATEMPERATURE SUSH THAT SUBSTANTIALLY ALL CARBON DIOXIDE IN SAID AIR HASBEEN REMOVED FROM THE AIR IN ITS PASSAGE THROUGH ITS PATH IN SAID SECONDZONE, MAINTAINING THE TEMPERATURE DIFFERENCE BETWEEN THE TEMPERATURE OFTHE AIR LEAVING AND THE WEIGHTED AVERAGE TEMPERATURE OF THE NITROGEN ANDOXYGEN ENTERING SAID SECOND ZONE SO THAT IT FALLS WITHIN THE RANGE ORFROM ABOUT 5* TO ABOUT 10* F., PASSING THE AIR FROM SAID SECOND ZONE TOTHE HIGH-PRESSURE STAGE OF A TWO-STAGE RECTIFICATION SYSTEM IN INDIRECTHEAT EXCHANGE RELATION WITH A RECTIFICATION PRODUCT FROM THELOW-PRESSURE STAGE OF SAID SYSTEM, WITHDRAWING A MINOR PORTION OF THENITROGEN FROM THE HIGH-PRESSURE STAGE, EXPANDING THE NITROGEN THUSWITHDRAWN TO COOL THE SAME, IMPARTING THE COLD THUS PRODUCED TO THERECTIFICATION PRODUCTS ENTERING THE LOW-PRESSURE STAGE, PERIODICALLYREVERSING THE FLOW OF AIR AND NITROGEN THROUGH THEIR RESPECTIVE PATHS INSAID ZONES, THE AIR UPON REVERSAL FLOWING THROUGH THE PATHS THROUGHWHICH HAD PREVIOUSLY FLOWED THE NITROGEN AND THE NITROGEN FLOWINGTHROUGH THE PATHS THROUGH WHICH HAD PREVIOUSLY FLOWED THE AIR, WHEREBYUPON EACH REVERSAL THE NITROGEN SUBSTANTIALLY COMPLETELY REMOVES THECARBON DIOXIDE DEPOSITED IN SAID SECOND ZONE DURING THE PRECEDING STEPOF THE PROCESS.